GP light duration - PeachParts Mercedes-Benz Forum

mach0415 · #1 10-05-2010, 06:08 PM

What is the generally acceptable duration for the GP light? Here in NC it has been getting down to 40s at night. The GP light is staying on for maybe 5 secs, but my SD is running rough on cold start as if some GPs are not on. Now, I have been letting them glow for 30-60 secs before cranking. Still rough idle until heat is built up. I know I need to ohm them out. Is a shorter duration due to less resistance from opens in one or more of the GPs?

Diesel911 · #2 10-05-2010, 07:00 PM

The Glow Plug Relay has a temp Sensor inside that is supposed to determing how long the Glow Plug Light stays on.
And, that is supposed to be an indicator as to when you can start
.
However, the Glow Plug Temp Sensor and Relay do not know the condition of your Engine.
When the Glow Plug Light goes out and you leave the Key in the Preglow Position the Glow Plug Relay will continue to function for another 20 seconds or so until you hear it click off.

When you start Cranking the Starter the Glow Plug Relay is again turned on until you are done Cranking the Starter.

I am gessing If you have a low milage Engine using the Glow Plug Light to indicate when you can start is OK. If you have some wear on your Engine you will need to decide how long you want to Glow.

Other major factors of the idle roughness is related to some Glow Plugs not working, Valve Adjustment/Compression and the Condition of the Injectors.

79Mercy · #3 10-05-2010, 07:01 PM

5-8 seconds for pencil plugs is plenty at that temperature. I recommend checking and replacing the bad glow plugs, and give the car a valve adjustment if you haven't preformed one in the past year. That will greatly help your cold starting ability.

EDIT: whats the condition of your fuel injectors, poor spray pattern and unbalanced pop pressures will cause rough cold idle.

Jeremy5848 · #4 10-05-2010, 07:35 PM

The preglow light almost always turns off too soon, especially when the engine is cold from sitting overnight. With a cold engine I always glow at least 10 seconds regardless of what the light does.

The duration of the preglow light is set internally by a time-delay circuit. This circuit has no idea whether any or all of the glow plugs are good or bad. Later models actually look at the engine temperature but I believe your model looks only at the ambient temperature (temperature in the preglow relay).

A second time-delay circuit turns off the glow plugs after about 30 seconds. If it isn't too noisy where you live, you should be able to hear the preglow relay click off. There's no point in waiting any longer as the plugs will then begin to cool. If the engine runs rough then it is likely that one or more plugs are bad.

Jeremy

tangofox007 · #5 10-05-2010, 08:46 PM

Quote:

Originally Posted by Jeremy5848

A second time-delay circuit turns off the glow plugs after about 30 seconds.

Unless the key is moved to start before that time, in which case the cycle terminates when the key is subsequently released from the start position. (Assumes that an OE type timer is installed.)

mach0415 · #6 10-06-2010, 05:18 AM

I did a valve adjustment back in April, replaced all filters twice - once for a Lubri-Moly diesel purge, checked timing chain stretch (4 degrees with no offset key) and replaced the glow plugs with cheap Autolites. This is most likely the culprit. I have heard a lot about their poorer quality over short periods of time. As far as I know, the injectors are original. I do not know about their spray pattern, etc. I will test GPs today to see whats up.

79Mercy · #7 10-06-2010, 08:33 AM

autolite=ick, Bosch or Beru glowplugs are the only acceptable glow plugs for a Mercedes diesel.

4 degrees of stretch is not to bad, i don't think that should cause an issue. However you need to check injection pump timing. It must be retarded since your chain is slightly stretched.

JiveTurkey · #8 10-06-2010, 12:41 PM

Quote:

Originally Posted by mach0415

and replaced the glow plugs with cheap Autolites.

This is likely your problem. A while back I bought new GP's for my 300SD thinking the cheap autolites would get the job done. I tested the resistance on all of them before installing them and they all tested fine. Unfortunately the car started more roughly with five autolites than with 3/5 oem bosch plugs. I bought Bosch GP's the same day.

Diesel911 · #9 10-06-2010, 12:52 PM

Quote:

Originally Posted by mach0415

I did a valve adjustment back in April, replaced all filters twice - once for a Lubri-Moly diesel purge, checked timing chain stretch (4 degrees with no offset key) and replaced the glow plugs with cheap Autolites. This is most likely the culprit. I have heard a lot about their poorer quality over short periods of time. As far as I know, the injectors are original. I do not know about their spray pattern, etc. I will test GPs today to see whats up.

Autolite Glow Plugs are more than just bad.
I installed new Autolite GPs in my Car when I first got the Car. I did not have A Glow Plug Reamer at the time.
6 Months later when I got a Glow Plug Reamer I decided to pull the Glow Plugs and ream the Carbon out of the Holes.

While the Autolites were working OK I barely got 2 of them out of the Cylinder head and had to use a Vicegrips to do that.
What had happen is that the tips of the Glow Plugs had swollen up slightly larger than the Hole!!!
If I had left them in longer I might have had to pull the Head to get them out.

Also, someone in or Forum Posted pics of Autolite Glow Plugs where it looks as if the tips had actuall exploded!!!

mach0415 · #10 10-06-2010, 04:25 PM

Yeah, I have seen Autolite glow plugs and what they do before on this forum. The only reason why I used them was because it was cheap, available, and I did not know how long this car would run, as it was new to me with what I though was a lot of miles. I got comfortable with it, and am now going to replace them with Bosch GPs.

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